Characterization and mechanisms of H₂O₂-induced contractions of pulmonary arteries

We studied H₂O₂-induced contractions of isolated rabbit intrapulmonary arteries mounted in standard tissue baths. All vessels were pretreated with a thromboxane A₂/prostaglandin H₂ receptor antagonist, SQ 29,548, to block immediate transient contractions to H₂O₂ and to isolate slowly developing sustained contractions. When exposed to H₂O₂ (0.1, 0.2, 0.3, 0.6, and 1.0 mM) for 30 min, vessels contracted in a concentration-dependent fashion between 0.1 and 0.3 mM H₂O₂; contractions at 0.6 and 1.0 mM H₂O₂ were not significantly different from those at 0.3 mM H₂O₂. During recovery (90 min) from H₂O₂ exposures, baseline tension was significantly greater, but active tension (10 μM phenylephrine) was significantly less for vessels previously exposed to 0.6 and 1.0 mM H₂O₂. Contractions to 0.3 mM H₂O₂ were not blunted by the following interventions: 1) endothelium rubbing, 2) incubation in Ca²⁺-free 100 μM ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'- tetraacetic acid (EGTA) Krebs-Ringer solution, 3) incubation in the Ca²⁺- free solution and depletion of ryanodine (20 μM)-sensitive Ca²⁺ stores, or 4) pretreatment with the protein kinase C inhibitor 1-(5- isoquinolinylsulfonyl)-3-methyl-piperazine (20 μM). However, contractions were depressed by ~50% when vessels were pretreated with the phospholipase C/serine esterase inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (50 μM). These results suggest that slow-developing contractions to H₂O₂ are concentration dependent and may result, in part, from activation of a serine esterase(s) and/or phospholipase C.